Strategies for CO2 storage in oceans. Redrawn from artwork of Sean Goddard, University of Exeter. After IPCC 2005.

Carbon Storage in the Deep Ocean

Published: November 9, 2010, 12:00 am

Updated: May 7, 2012, 6:11 pm

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Earth’s oceans are vast, covering over 70% of the planet’s surface at an average depth of 3800 meters. Injecting CO₂ into the deep ocean would isolate it from the atmosphere for several centuries and halve peak atmospheric concentrations. Eventually, after about 2 millennia, CO₂ concentrations in the atmosphere and oceans would come back into equilibrium. Therefore, oceans can offer long-term, but not permanent, storage.

Several mitigation strategies envision that electric power plants would pump captured CO₂ down pipelines into the ocean depths. Alternatively, tanker ships carrying CO₂ would release it at depth or would carry the CO₂ to platforms that release it at depth. Finally, some plants would convert CO₂ into CaCO₃ and dissolve it in seawater for release at depth.

The fate of CO₂ in water is complex, both in its dissociation into H₂CO₃, HCO₃ ^–, and CO₃ ^2– and in its transitions among gas, liquid, and solid phases. In oceans, as depth increases, hydrostatic pressure increases and temperature drops. Injected CO2 remains a gas until about 400 m depth, at which point it becomes a supercritical fluid (its density varies continuously with pressure, without a distinct phase change).

For reasons that are not well understood, a portion of the CO₂ at these depths forms hydrates. In a hydrate, a solid, crystalline cage of water molecules surrounds each CO₂ molecule (CO₂·6H₂O). Hydrates of CO₂, denser than seawater, tend to sink in the water column, whereas liquid CO₂, less dense than seawater at depths between 400 m and 2700 m, tends to rise. Below 3000 m, liquid CO₂ becomes denser than seawater and sinks. Injection of CO₂ into deep water, therefore, may establish CO₂ lakes in depressions on the sea bottom. Such lakes would delay the dispersion of CO₂ into the surrounding environment.

Large-scale injection of captured CO₂ into the oceans would influence sea life. High CO₂ concentrations themselves can interfere with aerobic respiration. Moreover, liquid CO₂ behaves as an organic solvent that may prove toxic to certain organisms. Finally, dissolution of CO₂ into its various bicarbonate and carbonate forms would acidify the surrounding water, and sea life would respond to these pH changes. Nonetheless, we have such limited knowledge about life in deep oceans that we cannot anticipate all of the consequences of massive CO₂ injections. [1]

Other carbon dioxide storage methods include converting CO₂ into mineral carbonates and pumping CO₂ into the deep sea.

[1] Nealson, K. (2006) Lakes of liquid CO2 in the deep sea. Proceedings of the National Academy of Sciences of the United States of America 103:13903-13904.

This is an excerpt from the book Global Climate Change: Convergence of Disciplines by Dr. Arnold J. Bloom and taken from UCVerse of the University of California.

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